1. Field of the Invention
The present invention relates, in general, to an apparatus and method for performing an electrochemical process on electrically conductive particles, and, in particular, to an electrolyzer and method for electrodeposition on electrically conducting particles.
2. Related Art
One of the more promising alternatives to conventional power sources in existence today is the metal/air fuel cell. These fuel cells have tremendous potential because they are efficient, environmentally safe and completely renewable. Metal/air fuel cells can be used for both stationary and motile applications, and are especially suitable for use in all types of electric vehicles.
Metal/air fuel cells and batteries produce electricity by electrochemically combining metal with oxygen from the air. Zinc, iron, lithium, and aluminum are some of the metals that can be used. Oxidants other than air, such as pure oxygen, bromine, or hydrogen peroxide can also be used. Zinc/air fuel cells and batteries produce electricity by the same electrochemical processes. But zinc/air fuel cells are not discarded like primary batteries. They are not slowly recharged like secondary batteries, nor are they rebuilt like xe2x80x9cmechanically rechargedxe2x80x9d batteries. Instead, zinc/air fuel cells are conveniently refueled in minutes or seconds by adding additional zinc when necessary. Further, the zinc used to generate electricity is completely recoverable and reusable.
The zinc/air fuel cell is expected to displace lead-acid batteries where higher specific energies are required and/or rapid recharging is desired. Further, the zinc/air fuel cell is expected to displace internal combustion engines where zero emissions, quiet operation, and/or lower maintenance costs are important.
In one example embodiment, the zinc xe2x80x9cfuelxe2x80x9d is in the form of particles. Zinc is consumed and releases electrons to drive a load (the anodic part of the electrochemical process), and oxygen from ambient air accepts electrons from the load (the cathodic part). The overall chemical reaction produces zincate or its precipitate zinc oxide, a non-toxic white powder. When all or part of the zinc has been consumed and, hence, transformed into zincate or zinc oxide, the fuel cell can be refueled by removing the reaction product and adding fresh zinc particles and electrolyte.
The zincate or zinc oxide (ZnO) product is typically reprocessed into zinc particles and oxygen in a separate, stand-alone recycling unit using electrolysis. The whole processing a closed cycle for zinc and oxygen, which can be recycled indefinitely.
In general, a zinc/air fuel cell system comprises two principal components: the fuel cell itself and a zinc recovery apparatus. The recovery apparatus is generally stationary and serves to supply the fuel cell with zinc particles, remove the zinc oxide, and convert it back into zinc metal fuel particles. A metal recovery apparatus may also be used to recover zinc, copper, or other metals from solution for any other purpose. In particular, a metal recovery apparatus may be used to economically recover metals from scrap or from processed ore.
The benefits of zinc/air fuel cell technology over rechargeable batteries such as lead-acid batteries are numerous. These benefits include very high specific energies, high energy densities, and the de-coupling of energy and power densities. Further, these systems provide rapid on-site refueling that requires only a standard electrical supply at the recovery apparatus. Still further, these systems provide longer life potentials, and the availability of a reliable and accurate measure of remaining energy at all times.
The benefits over internal combustion engines include zero emissions, quiet operation, lower maintenance costs, and higher specific energies. When replacing lead-acid batteries, zinc/air fuel cells can be used to extend the range of a vehicle or reduce the weight for increased payload capability and/or enhanced performance. The zinc/air fuel cell gives vehicle designers additional flexibility to distribute weight for optimizing vehicle dynamics.
The benefits of using an electrolyzer with a moving particulate bed for metal recovery from processed ore or scrap include the following: 1) The energy consumption per unit of metal produced can be far lower than with traditional techniques; 2) The apparatus can be run continuously without periodic labor intensive shutdowns for removing recovered metal in slab form, as with traditional techniques; 3) The particulate form of the metal produced is much more convenient to store, distribute, ship, and use than are the metal slabs produced using traditional apparatus.
The recovery apparatus uses an electrolyzer to reprocess dissolved zinc oxide into zinc particles for eventual use in the fuel cells (or, in the case of a metal processing or recovery application, into metal particles that can be conveniently stored, shipped, and introduced into metal refining, casting, or fabrication processes). The electrolyzer accomplishes this by electrodepositing zinc from the zinc oxide on electrically conducting particles. Fluidized bed electrolyzers and spouted bed electrolyzers are examples of two types of technologies used for the electrodeposition of metals on conducting particles (see for example U.S. Pat. No. 5,695,629, Nadkami et al.; xe2x80x9cSpouted Bed Electrowinning of Zinc: Part I, Juan Carlos Salas-Morales et al., Metall. Trans. B, 1997, vol. 28B, pp. 59-68; U.S. Pat. No. 4,272,333, Scott et al.; and U.S. Pat. No. 5,958,210, Siu et al.). In both a fluidized bed electrolyzer and spouted bed electrolyzer, the anodes are separated from the fluidized particles by a separator. The separator must be an ionic conductor but not an electrical conductor and must be resistant to erosion and dendrite growth for the electrolyzer to perform reliably. The dendrite problem is particularly difficult to avoid since if a single conducting particle becomes trapped in or on the separator, and if the particle remains in electrical contact with the bed of moving particles, it will grow through the separator toward the anode and cause an electrical short. At this point, the electrolyzer may have to be disassembled and rebuilt with a new separator. Another problem with some electrolyzers is the low volumetric efficiency or low space time yield of the device. In other words, a device of a given size does not produce enough metal per unit time to be economically viable or practical. This results from the fact that In a conventional xe2x80x9cplatexe2x80x9d electrolyzer the cathode is a zinc plate, which has a much lower surface area on which electrodeposition can take place than does a bed of particles occupying a similar volume. Therefore the yield of electrodeposited material per unit volume may be very low in a conventional flat plate system.
A problem with a traditional fluidized bed electrolyzer is the high pumping energy required to maintain the cathode particle bed in fluid motion, thereby decreasing the overall efficiency of the system. Yet another disadvantage of the traditional fluidized bed electrolyzer is the poor average electrical contact made by the fluidized cathode particles with the current collector, further reducing the energy efficiency of the system.
Thus, what is needed is an electrolyzer for electrodeposition on electrically conductive particles that maintains good electrical contact between the power supply and the conducting particles, does not require unacceptably high pumping power, and eliminates the need for a separator, thereby avoiding the aforementioned problems with separator erosion through contact with the moving particles, and the growth of dendritic particles which penetrate the separator and cause an electrical short between the anode and cathode. The electrolyzer should also have a high yield of electrodeposited material per unit volume.
Accordingly, the present invention eliminates the need for a separator in an electrolyzer for electrodeposition on electrically conductive particles, thereby avoiding separator erosion problems and short circuit problems caused by dendritic particle growth. The invention also has a high yield of electrodeposited material per unit volume.
The present invention provides an electrolyzer for electrodeposition onto electrically conductive particles. In one embodiment, the electrolyzer includes a cathode support including an upper surface with at least one dimension inclined at an angle relative to horizontal sufficient to allow gravitational forces to cause a bed of the electrically conductive particles to flow at a substantially uniform density and flow rate down the upper surface. The flowing bed of particles is the cathode. The cathode support is preferably, but not necessarily, planar. An electrical contact is made with the cathode (the bed of particles) either by the cathode support or by some other means, where the electrical contact can be connected to an electrical power supply. The cathode support includes an upper portion at which the particles enter the cathode support surface and a lower portion at which the particles exit the cathode support surface. In one embodiment, an anode is spaced from the cathode, without a separator therebetween, a distance sufficiently small to minimize resistance to ionic current flow between the anode and the particles and yet sufficiently large to allow clearance for the bed of electrically conductive particles flowing down the cathode support surface without sustained contact with the anode. This distance should be between 1 and 50 times the average diameter of the conductive particles, and preferably between 1 and 10 times the average diameter of the conductive particles. A recirculation line communicates the lower end of the cathode with the upper end of the cathode. A pump is interconnected with the recirculation line and adapted to transfer fluidized particles at the lower portion of the cathode to the top of the cathode.
Multiple embodiments of the invention are possible, including constructions in which the cathode support is an inclined plate, a helical surface, a spiral surface, a spinning funnel-shaped element, and a vibrating plate, and in which the force causing particle movement on the cathode support is gravity, a frictional force created by vibration, a centrifugal force, or some other force. In the embodiment in which the cathode support is an inclined surface, the cathode support may be made of any material that can chemically withstand the fluidizing liquid electrolyte and the abrasive action of the moving particle bed, and the surface of the cathode support should have a sufficiently low coefficient of friction to ensure the particle bed does not stop flowing down the inclined dimension of the cathode support. The angle of the inclined surface needs to be sufficiently steep to ensure constant motion of the particle bed but sufficiently shallow to keep the particle bed as dense as possible. The best range of angles depends upon several factors, including the coefficient of friction of the cathode support, the density and viscosity of the electrolyte, and morphology of the particles, and the type of metal. For electrodeposition of zinc onto zinc cut wire particles approximately 0.75 mm in diameter in 35% potassium hydroxide solution at 50xc2x0 C. with a 304 stainless steel cathode support with roughness xcex5/dp preferably being within the range 0xe2x89xa6xcex5/dpxe2x89xa610, and optimally, within the range 0xe2x89xa6xcex5/dpxe2x89xa60.1, acceptable angles were observed to be between about 10 and 45 degrees, with the best angles in the range of 20 to 25 degrees. In the foregoing, the parameter xcex5/dp is dimensionless, and comprises the ratio of xcex5, the height of the roughness, and dp, the particle diameter. The anode generally has a mesh construction. The anode is preferably substantially flat and parallel with the cathode support if the cathode support is substantially flat. The anode is preferably planar and parallel to the surface of the cathode particle bed so as to minimize the distance between the anode and the cathode at all points. The anode is supported by a current collector, and for applications in which a gas such as oxygen may be evolved such as the reduction of metals from metal oxides, an oxygen escape region is generally located between the anode and the current collector. A feed control mechanism is generally located near the upper portion of the cathode, and the feed control mechanism is adapted to control the flow rate and density of the bed of electrically conductive particles flowing down the cathode support. A feed reservoir is adapted to hold a supply of the electrically conductive particles. A receiving reservoir, which is preferably but not necessarily distinct from the feed reservoir, is adapted to receive the electrically conductive particles after they flow down the inclined surface of the cathode. The recirculation line communicates the receiving reservoir with the feed reservoir. A fluid tank is adapted to hold fluid used to fluidize the electrically conductive particles. A fluid bleed line communicates the feed reservoir with the fluid tank. A fluid supply line communicates the fluid tank with the receiving reservoir.
An additional aspect of the invention involves a method of electrodepositing metal on electrically conductive particles. In one embodiment, the method includes providing an electrolyzer with a particulate cathode and a cathode support having an upper surface inclined at an angle relative to horizontal sufficient to allow gravitational forces to cause a bed of the electrically conductive particles to flow at a substantially uniform density and flow rate down the upper surface. One embodiment of the cathode support includes an upper portion at which the particles enter the cathode surface and a lower portion at which the particles exit the cathode support surface. An anode is spaced from the particulate cathode, without a separator therebetween, a distance sufficiently small to minimize resistance to ionic current flow between the anode and the particles and yet sufficiently large to allow clearance for the bed of electrically conductive particles flowing down the cathode support surface without significant contact with the anode. A recirculation line communicates the lower end of the cathode with the upper end of the cathode. A pump is interconnected with the recirculation line and adapted to transfer particles at the lower portion of the cathode to the top of the cathode. One embodiment of the method further includes supplying the electrolyzer with electrically conductive particles and a liquid electrolyte containing dissolved metal ions (simple or complex); allowing gravitational forces to cause the electrically conductive particles to flow at a substantially uniform density and flow rate down the upper surface; electrodepositing metal from the reaction product on the electrically conductive particles as the particles flow down the inclined surface of the cathode support by providing an electrical current between the anode and particulate cathode; and recirculating electrically conductive particles from the lower portion of the cathode to the upper portion of the cathode using the pump.
Embodiments of the aspect of the invention described immediately above may include one or more of the following: The cathode support includes a construction selected from the group consisting of an inclined plate, an inclined non-planar surface, a helical surface, a spiral surface, a vibrating surface, and a funnel-shaped rotating surface. In the embodiment in which the cathode support is an inclined surface, the cathode support may be made of any material that can chemically withstand the fluidizing liquid electrolyte and the abrasive action of the moving particle bed, and the surface of the cathode support should have a sufficiently low coefficient of friction to ensure the particle bed does not stop flowing down the inclined dimension of the cathode support. The angle B of the inclined surface from horizontal needs to be sufficiently steep to ensure constant motion of the particle bed but sufficiently shallow to keep the particle bed as dense as possible. The best range of angles are between 5 degrees and 75 degrees and depends upon several factors, including the coefficient of friction of the cathode support, the density and viscosity of the electrolyte, and the density and morphology of the particles. For electrodeposition of zinc onto zinc cut wire particles approximately 0.75 mm in diameter in 35% potassium hydroxide solution at 50xc2x0 C. with a 304 stainless steel cathode support with a roughness xcex5/dp preferably falling within the range 0xe2x89xa6xcex5/dpxe2x89xa610, and most preferably within the range 0xe2x89xa6xcex5/dpxe2x89xa60.1, acceptable angles were observed to be between about 10 degrees and 45 degrees, with the best angles between about 20 degrees and 25 degrees. The anode generally has a mesh construction. The anode is preferably substantially flat and parallel with the cathode support if the cathode support is substantially flat. The anode is preferably planar and parallel to the surface of the cathode particle bed so as to minimize the distance between the anode and the cathode at all points. The anode is supported by a current collector, and for applications involving the reduction of metals from metal oxides, an oxygen escape region is generally located between the anode and the current collector, and the method further includes removing oxygen produced during electrodeposition from the oxygen escape region. A feed control mechanism is located near the upper portion of the cathode, the feed control mechanism is adapted to control the flow rate and density of the bed of electrically conductive particles flowing down the cathode, and the method further includes controlling the flow rate and density of the electrically conductive particles flowing down the cathode with the feed control mechanism. A feed reservoir is adapted to hold a supply of the electrically conductive particles, and the method includes supplying the electrolyzer with electrically conductive particles and a liquid electrolyte containing dissolved metal ions (simple or complex)at the feed reservoir. A receiving reservoir, which is preferably but not necessarily distinct from the feed reservoir, is adapted to receive the electrically conductive particles after they flow down the inclined surface of the cathode support. The recirculation line communicates the receiving reservoir with the feed reservoir, and the method includes recirculating electrically conductive particles from the receiving reservoir to the feed reservoir through the recirculation line. A fluid tank is adapted to hold fluid used to fluidize the electrically conductive particles. A fluid bleed line communicates the feed reservoir with the fluid tank, and the method further includes bleeding a portion of fluid supplied to the feed reservoir to the fluid tank using the fluid bleed line. A fluid supply line communicates the fluid tank with the receiving reservoir, and the method further includes supplying additional fluid to the receiving reservoir using the fluid supply line.